With the 18" twist I would expect that the bullet would have to be a maximum of about 350 grains. The length has more to do with it than the weight.

If it is an original gun I think you mean the chamber is .452" diameter. At .425 and a .410 bullet the maximum brass thickness that would allow you to chamber a round is .012"

You might try the Hornady 405 brass. It is a little thicker than Krag brass. This would bring the neck diameter up to around .435-.436" You could neck size in a 40/65 die or get one of the Lyman .40 neck dies. You'll be working the brass excessively and it will have a short life. You could get cases from Rocky Mountain or solder a short piece of brass tubing to the cases at the mouth end and turn it for the proper fit. The soldering temperature is less than the annealing temperature of the brass. You would need some tools to do this work. After fireforming the cases should work OK for blackpowder loads.

Maybe some other folks will have other ideas.

Are you sure of the bore size?  Layman's book lists the 40-60 1873 model as using a 423 bullet and the 1882 model as using a 417 bullet.  I have one of each and they follow this sizing.   
Bruce

Bruce & John, 

You will recall that sometimes one encounters a Model 1873 with a Model 1882 barrel or vice versa, or even a barrel that is actually right for the Model with an unexpected bore diameter (like my 1873 barrel in 40-40)  With Maynard, expect the unexpected!

Froggie

Your numbers don't make much sense. If the chamber is .425" and the bullet is .407" that leaves .009" max. for the brass thickness per side which is actually thin for most brass. Measure the fired case at the neck diameter. Measure the inside diameter of the fired case with a caliper to see what size it is. An original 40/60 Maynard chamber should measure about .450"-.452" at the neck. You should slug the bore from the breech end. Many of those barrels were choked some at the muzzle. 

Yes, a chamber can be sleeved without sleeving the entire barrel. If you've got a nice original barrel with a mint bore as you say it would be a shame to mess with it. Work with the brass & bullets to solve the problem. .417" bullets would be a custom mold or there are some folks who can open up the grooves on an existing mold to get the .417 size.

I forgot to mention: I use .40-70 Sharps STR reloading dies to reload my .40-60 Maynard cases.

See pg. 50 "SPG Lubricants BP Cartridge Reloading Primer" by Mike Venturino & Steve Garbe regarding the reloading of the .40-60 Maynard.

You can also use the .40-50 Sharps Straight dies as well to reload the .40-60 Maynard cases.

My loaded .40-65 Maynard with a 0.410" diameter bullets is  0.431" (outside) at the mouth and 0.452" (outside) at the web (against rim) in a case with a maximum length of the case of 2.200" (without bullet).

If your chamber length is 2.500" then you must be measuring down the chamber lead as well.